An electrophotographic image forming apparatus configured as a printer, a copying machine, a facsimile machine, a multi-functional machine, and the like writes an electrostatic latent image by irradiating a light onto a photosensitive drum (a typical example of an image bearing body) charged by a charging unit, develops the electrostatic latent image as a toner image (forming the toner image) by having a toner supplied to the photosensitive drum by a developing unit, transfers the toner image onto a predetermined recording material such as recording paper directly or on to the recording material through an intermediate transfer member such as an intermediate transfer belt, and ultimately fixes on the recording paper or the intermediate transfer member by heating the toner image with heating rollers provided in a fixing unit. In the above process, a predetermined voltage is applied to the charging unit. The applied voltage to the charging unit is hereinafter referred to as a charging applied voltage.
Some of such image forming apparatuses have an image forming mode selected and set out of a predetermined plurality of candidates and carry out an image forming process according to the operating conditions corresponding to set image forming mode. Furthermore, some apparatuses set a rotating speed of the photosensitive drum according to the selected image forming mode to ensure a constant image quality.
Some of such image forming apparatuses has an image forming mode selected and set out of a predetermined plurality of candidates and carry out an image forming processing according to the operating conditions corresponding to the set image forming mode. Furthermore, some apparatuses set a rotating speed of the photosensitive drum according to the selected image forming mode to ensure a constant image quality.
For example, an image forming apparatus having, as the image forming mode, a monochrome image forming mode of forming a monochrome image and a color image forming mode of forming a color image sets (controls) the rotating speed (circumferential velocity) of the photosensitive drum slower when the color image forming mode is selected than when the monochrome image forming mode is selected.
Another image forming apparatus having, as the image forming mode, a low-temperature and low-humidity mode to be selected when the temperature and the humidity of the ambience in which the photosensitive drum is installed are less than the predetermined level and other ambience modes (standard mode, high-temperature and high-humidity mode, etc.) to be selected in other cases sets the reference level of the charging applied voltage higher when the low-temperature and low-humidity mode is selected than when the other ambience modes are selected. The reason is as follow. Namely, since, in the low-temperature and low-humidity ambience, a phenomenon of the toner charge volume becoming greater occurs and a charge potential of the photosensitive drum becomes difficult to enhance, a sufficient image density can not be obtained by ordinary developing. Therefore, by setting the level of the charging applied voltage high, a sufficient image density can be ensured.
On the other hand, Japanese Laid-Open Patent Publication No. H08-179594 describes an image forming apparatus that counts the number of printed sheets (typed sheets) and sets the charging applied voltage at a rather high voltage initially and then gradually decreases the voltage as the number of printed sheets increases, while the number of printed sheets is still below a predetermined number of sheets, namely, until when the number of printed sheets has exceeded the predetermined number of sheets and charging capability of a charger roller has settled down to a constant level. This technology has an object of keeping a constant charged potential of a photosensitive drum irrespective of a degree of use of the charger roller and ambient conditions.
While a photosensitive layer is formed on the surface of the photosensitive drum, the photosensitive layer deteriorates due to abrasion as the number of times of use (namely, the number of times of image forming) of the photosensitive drum increases. This deterioration is hereinafter referred to as coat thinning. As the coat thinning of the photosensitive drum advances, electrostatic capacity of the photosensitive layer becomes larger and charge amount becomes greater, but if the electric field intensity remains the same, a decrease of the charge potential of the photosensitive drum adversely becomes larger due to the advance of the coat thinning. As a result, there was a problem that the volume of toner attached to the photosensitive drum decreases, that a toner image of low density is formed, and that image deficiency occurs.
A degree of coat thinning (amount of coat thinning) of the photosensitive drum is not only proportional to the number of times of use (may be the number of times of execution of image forming or the number of printed sheets) but also subject to the rotating speed (circumference velocity) of the photosensitive drum and ambient conditions such as temperature and humidity.
On the other hand, the technology described in Japanese Laid-Open Patent Publication No. H08-179594 is not intended to prevent the above-mentioned developing deficiency by appropriately setting the charging applied voltage in accordance with the degree of coat thinning of the photosensitive layer of the photosensitive drum.
In this connection, it is conceivable that, when the coat thinning is caused to the photosensitive drum, the developing deficiency can be prevented by adjusting the charging applied voltage according to the degree of the coat thinning. Then it is conceivable to apply the technology described in Japanese Laid-Open Patent Publication No. H08-179594 as measures against the developing deficiency attributable to the coat thinning of the photosensitive drum and to adjust the charging applied voltage according to the cumulative number of printed sheets. In this case, however, the following problems remain.
As described above, the degree of coat thinning of the photosensitive drum is also subject to factors other than the number of printed sheets. As a result, there remains a problem that when other factors than the number of printed sheets have changed, the technology described in Japanese Laid-Open Patent Publication No. H08-179594 can not set the appropriate charging applied voltage corresponding to the degree of coat thinning and can not secure the appropriate image density.